The following code prints "KeyError()", which is obviously wrong and rather baffling. Just passing an exception object around *as an object* should not trigger implicit exception chaining!

If you replace the async functions with regular functions then it prints "None", as expected.

Checked on 3.5, 3.6, and more-or-less current master -- bug is present in all 3.

------------

async def f():
    return ValueError()

async def g():
    try:
        raise KeyError
    except:
        value_error = await f()
        print(repr(value_error.__context__))

try:
    g().send(None)
except StopIteration:
    pass

What a bizarre bug.  Created a PR with a fix.  

Nathaniel, please put me in a nosy list when you submit issues about generators/coroutines.

I am wondering how much of other uses of PyErr_SetObject() are affected by similar bugs?

> I am wondering how much of other uses of PyErr_SetObject() are affected by similar bugs?

Only when we use an exception to carry a return value.  AFAIK StopIteration is the only such case.